I was once told that playing a guitar with greasy fingers was actually good for the strings. However, this seems a bit counter-intuitive. Is there any truth behind this claim?
Disclaimer: I'm not an expert in this, yet try to make an attempt to summarize what I know on this subject.
What does Grease consist of?
Grease consists mainly of dirt, dust and sweat.
Dirt and dust consist of human skin cells, plant pollen, human and animal hairs, textile fibers, paper fibers, minerals from outdoor soil, and many other materials which may be found in the local environment. Most of these are soft so for these it's rather a concern about how they make your string look...
Sweat consiss mostly out of water (H²O), some urea (A water-soluble organic compound, CO(NH²)², formed by the metabolism of proteins.) and a bit of salt (NaCl). Water isn't harmful at all, urea isn't either (unless you somehow manage to nitrate in which case it comes highly explosive). So our main concern should be the salt in terms of the life span of your strings, as it is a corrosive material...
Let's see how these affect different types of guitar strings.
Here, your main concern is how your strings would look, as dirt/dust/sweat can stay on your strings.
Nylon strings are corrosion-resistant and doesn't have a problem with salt.
For example, imagine a guitar that has been in your basement for a very long time, if you blow at the strings a lot of dust will come of. Unless you properly stored it inside a bag... :)
What applies for the nylon strings also applies here.
But an extra concern here is that salt is a corrosive material against metal, so technically, cleaning your guitar after practicing and performance can help to increase the lifetime of your strings.
Is it worth cleaning my guitar strings? How do I do it?
I would say, yes, because they will look less dirty and more shiny. And for metal strings you have the added benefit that the lifetime could be longer, so it's certainly something you could consider doing.
The question is a bit vague as to what good and bad mean, the answers here are talking about lifespan, but it is also worth considering that as strings get dirtier it changes the sound (note - I'm an electric guitar player, things may be a little different for acoustic/classical, but I expect they'll be similar). Try comparing a brand new set of strings to an older set and you'll hear that the new clean ones have more jangly top end. Most people seem to prefer the sound of new strings, so keeping em clean would be a good thing. Me, I prefer older strings myself, so a good bit of skank makes me happy :) That said, grungier strings don't intonate so well either. Ho hum.
Definitely bad. Grease is sticky, ideal for lubricating things which need high viscosity. it holds particles of grit, dust, anything, and can hold moisture in.
Guitar strings should be kept smooth, away from moisture or anything which could corrode or rust them, and you don't want grease or grit on your fretboard.
So stick to a wipe with a dry cloth, Fast Fret and your strings will last much longer.
Oil prevents metal from oxidizing. Salts are corrosive. Sweat is both!
Overall, I'm pretty sure the salt wins out. My guitar strings always do worse when I don't keep them clean, and I've always been told that they should be cleaned after playing.
I wouldn't recommend oiling your guitar strings either, though. It'll get into the wood and make it gross, as well as altering the feel and sound of the strings. It's best just to keep them clean.